The invention shall be described, for greater facility, with reference to the processing of television signals without any limitation being thereby entailed in the scope of the invention. Among the television signals used to display images on cathode ray screens, there are the known vertical scanning signals, as well as horizontal scanning signals. These scanning signals are derived from vertical and horizontal synchronization signals. The scanning signals are applied to deflection coils of one or more guns projecting electrons from a cathode towards a light-emitting screen. To form an image, the scanning of the screen is thus prompted to form a certain number of lines stacked on one another from top to bottom.
Depending on the resolution envisaged, as well as on the display standards used in the different regions of the world, the number of lines displayed on the screen varies. Whereas, at present, there are known images with 420 lines or 625 lines, the new standards provide for 1024 lines or even 2048 lines. Furthermore, the refresh frequency of the images on the screen, which is 25 images per second, also depends on the standards and qualities sought. For example, 100 images per second are envisaged in order to avoid the effects of flicker. Multi-purpose scanning signal generators are therefore made to cope with these different standards.
These scanning signal generators have to generate saw-toothed scanning signals from synchronization signals, the period of these saw-toothed scanning signals being equal to the period of the corresponding synchronization signal, and their maximum amplitude being constant whatever this period. There are vertical synchronization signals and horizontal synchronization signals respectively associated with the vertical and horizontal scanning signals. These synchronization signals contain information elements on time related to the beginning of a line and the beginning of a frame (or half-frame). The period of these synchronization signals therefore varies as a function of the refresh frequency of the images on the screen and as a function of the number of lines displayed on the screen.
Until now, it is only analog circuits such as the TDA 9106 monitor circuit from SGS Thomson Microelectronics that fulfil this function. However, such circuits have an uncertainty of about 1% with regard to the maximum amplitude of the scanning signals at each change in frequency of the synchronization signal. Thus, for two different scanning frequencies, the maximum amplitude of the two scanning signals may be different. The implementation of such a circuit therefore requires a prior setting of the monitor according to the refresh frequency. At each change in standard, for example from VGA to SVGA, it is necessary to readjust the button for the vertical positioning of the image.